of: Allow selection of OF_DYNAMIC and OF_OVERLAY if OF_UNITTEST

Currently OF_DYNAMIC and OF_OVERLAY are not visible to the user, and are
selected automatically only when needed.

Allow them to be enabled manually to improve device tree unit test
coverage.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Grant Likely <grant.likely@linaro.org>